Serine/threonine protein phosphatases: Multi-purpose enzymes in control of defense mechanisms

Depending on the threat to a plant, different pattern recognition receptors, such as receptor-like kinases, identify the stress and trigger action by appropriate defense response development.^1,2 The plant immunity system primary response to these challenges is rapid accumulation of phytohormones, such as ethylene (ET), salicylic acid (SA), and jasmonic acid (JA) and its derivatives. These phytohormones induce further signal transduction and appropriate defenses against biotic threats.^3,4 Phytohormones play crucial roles not only in the initiation of diverse downstream signaling events in plant defense but also in the activation of effective defenses through an essential process called signaling pathway crosstalk, a mechanism involved in transduction signals between two or more distinct, “linear signal transduction pathways simultaneously activated in the same cell.”⁵     

Strigolactones' ability to regulate root development may be executed by induction of the ethylene pathway

The newly defined phytohormones strigolactones (SLs) were recently shown to act as regulators of root development. Their positive effect on root-hair (RH) elongation enabled examination of their cross talk with auxin and ethylene. Analysis of wild-type plants and hormone-signaling mutants combined with hormonal treatments suggested that SLs and ethylene regulate RH elongation via a common regulatory pathway, in which ethylene is epistatic to SLs. The SL and auxin hormonal pathways were suggested to converge for regulation of RH elongation; this convergence was suggested to be mediated via the ethylene pathway, and to include regulation of auxin transport.Key words: strigolactone, auxin, ethylene, root, root hair, lateral rootStrigolactones (SLs) are newly identified phytohormones that act as long-distance shoot-branching inhibitors (reviewed in ref. ¹). In Arabidopsis, SLs have been shown to be regulators of root development and architecture, by modulating primary root elongation and lateral root formation.^2,3 In addition, they were shown to have a positive effect on root-hair (RH) elongation.² All of these effects are mediated via the MAX2 F-box.^2,3In addition to SLs, two other plant hormones, auxin and ethylene, have been shown to affect root development, including lateral root formation and RH elongation.^4–6 Since all three phytohormones (SLs, auxin and ethylene) were shown to have a positive effect on RH elongation, we examined the epistatic relations between them by examining RH length.⁷ Our results led to the conclusion that SLs and ethylene are in the same pathway regulating RH elongation, where ethylene may be epistatic to SLs.⁷ Moreover, auxin signaling was shown to be needed to some extent for the RH response to SLs: the auxin-insensitive mutant tir1-1,⁸ was less sensitive to SLs than the wild type under low SL concentrations.⁷On the one hand, ethylene has been shown to induce the auxin response,^9–12 auxin synthesis in the root apex,^11,12 and acropetal and basipetal auxin transport in the root.^4,13 On the other, ethylene has been shown to be epistatic to SLs in the SL-induced RH-elongation response.⁷ Therefore, it might be that at least for RH elongation, SLs are in direct cross talk with ethylene, whereas the cross talk between SL and auxin pathways may converge through that of ethylene.⁷ The reduced response to SLs in tir1-1 may be derived from its reduced ethylene sensitivity;^7,14 this is in line with the notion of the ethylene pathway being a mediator in the cross talk between the SL and auxin pathways.The suggested ethylene-mediated convergence of auxin and SLs may be extended also to lateral root formation, and may involve regulation of auxin transport. In the root, SLs have been suggested to affect auxin efflux,^3,15 whereas ethylene has been shown to have a positive effect on auxin transport.^4,13 Hence, it might be that in the root, the SLs'' effect on auxin flux is mediated, at least in part, via the ethylene pathway. Ethylene''s ability to increase auxin transport in roots was associated with its negative effect on lateral root formation: ethylene was suggested to enhance polar IAA transport, leading to alterations in the quantity of auxin that unloads into the tissues to drive lateral root formation.⁴ Under conditions of sufficient phosphate, SL''s effect was similar to that of ethylene: SLs reduced the appearance of lateral roots; this was explained by their ability to change auxin flux.³ Taken together, one possibility is that the SLs'' ability to affect auxin flux and thereby lateral root formation in the roots is mediated by induction of ethylene synthesis.To conclude, root development may be regulated by a network of auxin, SL and ethylene cross talk.⁷ The possibility that similar networks exist elsewhere in the SLs'' regulation of plant development, including shoot architecture, cannot be excluded.     

Sublethal concentrations of salicylic acid decrease the formation of reactive oxygen species but maintain an increased nitric oxide production in the root apex of the ethylene-insensitive Never ripe tomato mutants

The pattern of salicylic acid (SA)-induced production of reactive oxygen species (ROS) and nitric oxide (NO) were different in the apex of adventitious roots in wild-type and in the ethylene-insensitive Never ripe (Nr) mutants of tomato (Solanum lycopersicum L. cv Ailsa Craig). ROS were upregulated, while NO remained at the control level in apical root tissues of wildtype plants exposed to sublethal concentrations of SA. In contrast, Nr plants expressing a defective ethylene receptor displayed a reduced level of ROS and a higher NO content in the apical root cells. In wild-type plants NO production seems to be ROS(H₂O₂)-dependent at cell death-inducing concentrations of SA, indicating that ROS and NO may interact to trigger oxidative cell death. In the absence of significant ROS accumulation, the increased NO production caused moderate reduction in cell viability in root apex of Nr plants exposed to 10⁻³ M SA. This suggests that a functional ethylene signaling pathway is necessary for the control of ROS and NO production induced by SA.Key words: ethylene receptor mutant, never ripe, nitric oxide, reactive oxygen species, root apex, salicylic acid, tomatoSeveral signal molecules, including salicylic acid (SA) have been implicated in the response of plants to biotic^1–3 and abiotic stressors.^4–6 SA was identified as a central regulator of local defense against (hemi)biotophic pathogens inducing a hypersensitive response (HR), which is characterized by the development of lesions that restrict pathogen spread. It has also emerged as a possible signaling component involved in the activation of certain plant defense responses in non-infected part of the plants establishing the systemic acquired resistance (SAR).⁷The SA-induced biotic and abiotic stress adaptation most likely involves reactive oxygen species (ROS) and nitric oxide (NO) in primary signaling events that activate multiple signal transduction pathways. SA-induced ROS is required for the activation of antioxidant defense mechanisms⁴ and if the generation of ROS exceeds the capacity of antioxidant systems, the cells die.⁸ NO is another important player that is required for the induction of defense mechanisms⁹ or for ROS-induced cell death.¹⁰Accumulation of SA, and two other plant hormones, ethylene (ET) and jasmonic acid (JA) are intimately associated with the initiation or spread of cell death. In HR SA and ROS have been proposed to be on a positive feedback loop that amplifies signals and leads to programmed cell death (PCD). Ethylene caused increased spreading of cell death, while lesion containment can be achieved by JA through decreasing the sensitivity of the cells to ethylene and through the suppression of SA biosynthesis and signaling.⁸Ethylene evolution is associated with diverse physiological processes such as leaf and flower senescence, abscission of organs and fruit ripening.¹¹ The biosynthesis of ethylene is stimulated by a variety of abiotic and biotic stress factors. Ethylene overproducing mutants (eto1 and eto3) of Arabidopsis were found to be more sensitive to O₃, an abiotic stressor which induces ROS-dependent cell death.¹² Cadmium-induced cell death was also accompanied by increased production of ethylene and simultaneously by H₂O₂ accumulation in tomato cell suspension, and based on the effect of specific inhibitors of ethylene biosynthesis and action the authors concluded that the cell death process required H₂O₂ production and a functional ethylene signaling pathway.¹³ Ethylene signaling is also required for the susceptible disease response of tomato plants infected with Xanthomonas campestris pv vesicatoria.¹⁴ It was found that the accumulation of SA and increased production of ethylene were important components of the disease symptoms of this pathogen in wild-type plants, while in Never ripe (Nr) mutants, which have a non-functional ethylene receptor, the infected plants failed to accumulate SA, produced less ethylene, and the leaves exhibited reduced necrotic lesions.It has been also shown that SA enhances NO synthesis in a dose-dependent manner.¹⁵ ROS, such as ^·O₂⁻ and H₂O₂ as well as NO can act together in the cell death regulation and propagation.^8,16 The compartment-specific (down)regulation of ROS can be controlled by NO, accordingly, ROS and NO homeostasis may be essential for the induction or for the avoidance of cell death.     

Is,indeed, the prion protein a Harlequin servant of “many” masters?

Tens of putative interacting partners of the cellular prion protein (PrP^C) have been identified, yet the physiologic role of PrP^C remains unclear. For the first time, however, a recent paper has demonstrated that the absence of PrP^C produces a lethal phenotype. Starting from this evidence, here we discuss the validity of past and more recent literature supporting that, as part of protein platforms at the cell surface, PrP^C may bridge extracellular matrix molecules and/or membrane proteins to intracellular signaling pathways.Key words: prion protein, PrP^C, extracellular matrix, cell adhesion molecules, neuritogenesis, p59fyn, Ca²⁺Initially, the discovery that the prion protein was the major, if not the unique, component of the prion agent causing transmissible spongiform encephalopathies (TSE)¹ has placed the protein in an extremely unfavorable light. Thereafter, however, a wealth of evidence has supported the notion that the protein positively influences several aspects of the cell physiology, and that its duality—in harboring both lethal and beneficial potentials—could be rationalized in terms of a structural switch. Indeed, the protein exists in at least two conformational states: the cellular, α helix-rich isoform, PrP^C, and the prion-associated β sheet-rich isoform, PrP^Sc.² If it is now unquestionable that the presence of PrP^C in the cell is mandatory for prion replication and neurotoxicity to occur,^3,4 nonetheless its physiologic function is still debatable, despite the long lasting effort, and the numerous, frequently genetically advanced, animal and cell model systems dedicated to the issue. From these studies the picture of an extremely versatile protein has emerged, whereby PrP^C acts in the cell defense against oxidative and apoptotic challenges, but also in cell adhesion, proliferation and differentiation, and in synaptic plasticity.^5,6 In an effort to converge these multiple propositions in an unifying functional model, different murine lines devoid of PrP^C have been studied. These animals, however, displayed no obvious phenotype,^7–9 suggesting that either PrP^C is dispensable during development and adult life or that compensative mechanisms mask the loss of PrP^C function in these paradigms. Thus, identifying the exact role of PrP^C in the cell would not only resolve an important biological question, but would also help elucidate the cellular steps of prion pathogenesis necessary for designing early diagnostic tools and therapeutic strategies for TSE.As is often the case, the employment of a model system unprecedented in prion research has recently disclosed a most interesting scenario with regards to PrP^C physiology, having unravelled, for the first time, a lethal phenotype linked to the absence of the protein.¹⁰ The paradigm is the zebrafish, which expresses two PrP^C isoforms (PrP1 and PrP2). Similarly to mammalian PrP^C, they are glycosylated and attached to the external side of the plasma membrane through a glycolipid anchor. PrP1 and PrP2 are, however, expressed in distinct time frames of the zebrafish embryogenesis. Accordingly, the knockdown of the PrP1, or PrP2, gene very early in embryogenesis impaired development at different stages, bypassing putative compensatory mechanisms. By focusing on PrP1, Malaga-Trillo et al. showed that the protein was essential for cell adhesion, and that this event occurred through PrP1 homophilic trans-interactions and signaling. This comprised activation of the Src-related tyrosine (Tyr) kinase p59fyn, and, possibly, Ca²⁺ metabolism, leading to the regulation of the trafficking of E-cadherin, a member of surface-expressed cell adhesion molecules (CAMs) responsible for cell growth and differentiation.¹¹ It was also reported that overlapping PrP1 functions were performed by PrP^Cs from other species, while the murine PrP^C was capable to replace PrP1 in rescuing, at least in part, the knockdown developmental phenotype. Apart from providing the long-sought proof for a vital role of PrP^C, the demonstration that a mammalian isoform corrected the lethal zebrafish phenotype strongly reinforces previous results—mainly obtained in a variety of mammalian primary neurons and cell lines—pointing to a functional interplay of PrP^C with CAMs, or extra cellular matrix (ECM) proteins, and cell signaling, to promote neuritogenesis and neuronal survival. A revisit of these data is the main topic of the present minireview.As mentioned, the capacity of PrP^C to act as a cell adhesion, or recognition, molecule, and to entertain interactions with proteins implicated in growth and survival, has already been reported for the mammalian PrP^C. A case in point is the interaction, both in cis- and trans-configurations, with the neuronal adhesion protein N-CAM12 that led to neurite outgrowth.¹³ Like cadherins, N-CAM belongs to the CAM superfamily. Following homo- or heterophylic interactions, it can not only mediate adhesion of cells, or link ECM proteins to the cytoskeleton, but also act as a receptor to transduce signals ultimately resulting in modulating neurite outgrowth, neuronal survival and synaptic plasticity.¹¹ Another example is the binding of PrPC to laminin, an ECM heterotrimeric glycoprotein, which induced neuritogenesis together with neurite adhesion and maintenance,^14,15 but also learning and memory consolidation.¹⁶ Further, it has been described that PrPC interacted with the mature 67 kDa-receptor (67LR) (and its 37 kDa-precursor) for laminin, and with glycosamminoglycans (GAGs), each of which is involved in neuronal differentiation and axon growth.^17–21 More recently, Hajj et al.²² have reported that the direct interaction of PrP^C with another ECM protein, vitronectin, could accomplish the same process, and that the absence of PrP^C could be functionally compensated by the overexpression of integrin, another laminin receptor.²³ Incidentally, the latter finding may provide a plausible explanation for the absence of clear phenotypes in mammalian PrP-null paradigms. By exposing primary cultured neurons to recombinant PrPs, others have shown that trans-interactions of PrP^C are equally important for neuronal outgrowth,^24,25 including the formation of synaptic contacts.²⁵ Finally, it has been demonstrated that the binding of PrP^C with the secreted co-chaperone stress-inducible protein 1 (STI1) stimulated neuritogenesis.²⁶ This same interaction had also a pro-survival effect, as did the interaction of PrP^C with its recombinant form.²⁴ Notably, the involvement of PrP^C in cell protection has been heightened by experiments with whole animals. By applying transient or permanent focal cerebral ischemia to the animals, it was found that their reduced brain damage correlated with spontaneous or adenoviral-mediated, upregulation of PrP^C,^27–29 (reviewed in ref. ³⁰), and that PrP^C deficiency aggravated their ischemic brain injury.^30,31 Thus, now that data are available from phylogenetically distant paradigms (zebrafish and mammalian model systems), it acquires more solid grounds the advocated engagement of PrP^C in homo/heterophilic cis/trans interactions to trigger signaling events aiming at neuronal—or, in more general terms, cell—survival and neuritogenesis. The latter notion is consistent with the delayed maturation of different types of PrP^C-less neurons, observed both in vitro and in vivo.^32,33If one assumes that the interaction of PrP^C with multiple partners (45 for PrP^C and PrP^Sc, as reviewed in Aguzzi et al.,⁵ or 46 considering the homophylic interaction) are all functionally significant, the most immediate interpretation of this “sticky” behavior entails that PrP^C acts as a scaffolding protein in different membrane protein complexes.^5,6 Each complex could then activate a specific signaling pathway depending on the type and maturation of cells, the expression and glycosylation of PrP^C, and availability of extra- and intra-cellular signaling partners. At large, all these signals have been shown to be advantageous to the cell. However, because in a cell only a subtle line divides the “good” from the “bad,” instances can be envisioned in which a pro-life signal turns into a pro-death signal. A typical example of this possibility is glutamate excitotoxicity resulting in dangerous, glutamate receptor-linked, Ca²⁺ overload. Likewise, an excessive or over-stimulated signal elicited by PrP^C, or by the putative complex housing the protein could become noxious to the cell. This possibility may explain why the massive expression of PrP^C caused degeneration of the nervous system,³⁴ and of skeletal muscles,^34,35 in transgenic animals. More intriguing is the finding that—in a mouse line expressing anchorless PrP^C—PrP^Sc was capable to replicate without threatening the integrity of neurons.³⁶ This may suggest that native membrane-bound PrP^C acts as, or takes part into, a “receptor for PrP^Sc”, and that lasting PrP^Sc-PrP^C interactions distort the otherwise beneficial signal of the protein/complex and cause neurodegeneration.³⁷ Consistent with this hypothesis is the finding that the in vivo antibody-mediated ligation of PrP^C provoked apoptosis of the antibody-injected brain area.³⁸ Speculatively, the action of N-terminally, or N-proximally truncated PrPs whose expression in PrP-less transgenic mice induced extensive neurodegeneration,^39–41 may be traced back to the same hyper-activation of PrP^C signaling. Possibly, this may hold true also for the synaptic impairment that, recorded only in PrP^C-expressing neurons, was attributed to the binding of amyloid beta (Aβ) peptide oligomers implicated in Alzheimer disease, to PrP^C.^42,43But which is (are) the cellular signaling pathway(s) conveyed by the engagement of PrP^C in different signaling complexes? In line with its multifaceted behavior, several intracellular effectors have been proposed, including p59fyn, mitogen-activated kinases (MAPK) Erk1/2, PI3K/Akt and cAMP-PKA. p59fyn is the most reported downstream effector, suggesting that, in accordance with its behavior, p59fyn could serve as the sorting point for multiple incoming and outgoing signals also in the case of PrP^C. The initial evidence of the PrP^C-p59fyn connection came from cells subjected to antibody-mediated cross-linking of PrP^C.⁴⁴ Later, it was shown that the PrP^C-p59fyn signal converged to Erk1/2 through a pathway dependent on (but also independent of) reactive oxygen species generated by NADPH oxidase.⁴⁵ A PrP^C-dependent activation of p59fyn^13,25 and Erk1/2 (but also of PI3K and cAMP-PKA)²⁴ was evident in other neuronal cell paradigms and consistent with the almost ubiquitous expression of PrP^C, in non-neuronal cells such as Jurkat and T cells.⁴⁶ Not to forget that in zebrafish embryonic cells activated p59fyn was found in the same focal adhesion sites harboring PrP1.¹⁰ Regarding the activation of the ERK1/2 pathway promoted by the PrP^C-STI1 complex, and leading to neuritogenesis, the role of p59fyn was not investigated.²⁶ The same holds true for the transduction of a neuroprotective signal by the PrP^C-STI1 complex involving the cAMP-PKA pathway.²⁶ Interestingly, this is not the only example reporting that engagement of PrP^C activates simultaneously two independent pathways. In fact, possibly after transactivating the receptor for the epidermal growth factor, the antibody-mediated clustering of PrP^C was shown to impinge on both the Erk1/2 pathway, and on a protein (stathmin) involved in controlling microtubule dynamics.⁴⁷Yet, if p59fyn is implicated in mammalian PrP^C-activated signaling cascade, a protein linking extracellular PrP^C to p59fyn is needed, given the attachment of the enzyme to the inner leaflet of the plasma membrane through palmitoylated/myristoylated anchors. In this, the PrP^C partner N-CAM (isoform 140) seems ideal to fulfill the task, given that p59fyn is part of N-CAM-mediated signaling. Indeed, after recruitment of N-CAM to lipid rafts—which may also depend on PrP^C,¹³—together with the receptor protein Tyr phosphatase α (RPTPα), the Tyr-phosphate removing activity of RPTPα allows the subsequent activation of p59fyn through an autophosphorylation step.⁴⁸ This event recruits and activates the focal adhesion kinase (FAK),¹¹ another non-receptor Tyr kinase. Finally, formation of the FAK-p59fyn complex triggers neuritogenesis through both Erk1/2 and PI3K/Akt pathways.^49,50 Parenthetically, the FAK-p59fyn and PI3K/Akt connection would be suitable to explain why aggravation of ischemic brain injury in PrP-deficient brains was linked to a depressed Akt activation.³¹ FAK-p59fyn complex, however, may be also involved in the signal triggered by the still mysterious PrP^C partner, 67LR. This protein was reported not only to act as a laminin receptor but also to facilitate the interaction of laminin with integrins,⁵¹ thereby possibly activating (through integrins) FAK-p59fyn-regulated pathways.⁴⁹ Conversely, other data have supported the candidature of caveolin-1 for coordinating the signal that from PrP^C reaches Erk1/2 through p59fyn.^44,45,52 Further scrutiny of this route has shown that it comprised players such as laminin and integrins (upstream), FAK-p59fyn, paxillin and the Src-homology-2 domain containing adaptor protein (downstream), and that caveolin-1, a substrate of the FAK-p59fyn complex, facilitated the interaction of these signaling partners by recruiting them in caveolae-like membrane domains.⁵³For the relevance they bear, we need to acknowledge recent propositions supporting the commitment of PrP^C with proteins whose function is unrelated from the above-mentioned cell adhesion or ECM molecules; namely, the β-site amyloid precursor protein (APP) cleaving enzime (BACE1) and the N-methyl-D-aspartate (NMDA)-receptor. BACE1 is a proteolytic enzyme involved in Aβ production. It has been shown that overexpressed PrP^C restricted, while depletion of PrP^C increased the access of BACE1 to APP, possibly because PrP^C interacts with BACE1 via GAGs.⁵⁴ Thus, native PrP^C reduces the production of Aβ peptides. A beneficial effect of PrP^C was also highlighted by Khosravani et al.⁵⁵ showing that, by physically associating with the subunit 2D of the NMDA-receptor, PrP^C attenuated neuronal Ca²⁺ entry and its possible excitotoxic effect. This clear example for the control of PrP^C on Ca²⁺ metabolism is particularly intriguing in light of previous reports linking Ca²⁺ homeostasis to PrP^C pathophysiology (reviewed in ref. ⁵⁶). Also, it is important to mention that a few partners of PrP^C or downstream effectors may initiate signals that increase intracellular Ca²⁺, and that, in turn, local Ca²⁺ fluctuations regulate some of the afore-mentioned pathways.^11,49,57,58In conclusion, although still somehow speculative, the implication of Ca²⁺ in PrP^C-dependent pathways raises the possibility that the different input signals originating from the interaction of PrP^C with diverse partners may all converge to the universal, highly versatile Ca²⁺ signaling. Were indeed this the case, then clearly the acting of PrP^C as Harlequin, the famous character of the 18^th century Venetian playwright Carlo Goldoni, who struggles to fill the orders of two masters, would be merely circumstantial.     

Cell Migration from Baby to Mother

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further understanding of the ability of fetal cells to cross both the placental and blood-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Here we discuss hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.Key Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migrationMicrochimerism is the presence of a small population of genetically distinct and separately derived cells within an individual. This commonly occurs following transfusion or transplantation.^1–3 Microchimerism can also occur between mother and fetus. Small numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the mother (fetomaternal)^4–7 and from the mother to the fetus.^8–10 Similar exchange may also occur between monochorionic twins in utero.^11–13 There is increasing evidence that fetomaternal microchimerism persists lifelong in many child-bearing women.^7,14 The significance of fetomaternal microchimerism remains unclear. It could be that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could be a mechanism by which the fetus ensures maternal fitness in order to enhance its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed understanding of the biology of microchimeric fetal cells may also advance progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.Trophoblasts were the first zygote-derived cell type found to cross into the mother. In 1893, Schmorl reported the appearance of trophoblasts in the maternal pulmonary vasculature.¹⁵ Later, trophoblasts were also observed in the maternal circulation.^16–20 Subsequently various other fetal cell types derived from fetal blood were also found in the maternal circulation.^21,22 These fetal cell types included lymphocytes,²³ erythroblasts or nucleated red blood cells,^24,25 haematopoietic progenitors^7,26,27 and putative mesenchymal progenitors.^14,28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,^29–31 trophoblast release does not appear to occur in all pregnancies.³² Likewise, in mice, fetal cells have also been reported in maternal blood.^33,34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.³⁵     

Auxin acts independently of DELLA proteins in regulating gibberellin levels

Shoot elongation is a vital process for plant development and productivity, in both ecological and economic contexts. Auxin and bioactive gibberellins (GAs), such as GA₁, play critical roles in the control of elongation,^1–3 along with environmental and endogenous factors, including other hormones such as the brassinosteroids.^4,5 The effect of auxins, such as indole-3-acetic acid (IAA), is at least in part mediated by its effect on GA metabolism,⁶ since auxin upregulates biosynthesis genes such as GA 3-oxidase and GA 20-oxidase and downregulates GA catabolism genes such as GA 2-oxidases, leading to elevated levels of bioactive GA₁.⁷ In our recent paper,¹ we have provided evidence that this action of IAA is largely independent of DELLA proteins, the negative regulators of GA action,^8,9 since the auxin effects are still present in the DELLA-deficient la cry-s genotype of pea. This was a crucial issue to resolve, since like auxin, the DELLAs also promote GA₁ synthesis and inhibit its deactivation. DELLAs are deactivated by GA, and thereby mediate a feedback system by which bioactive GA regulates its own level.¹⁰ However, our recent results,¹ in themselves, do not show the generality of the auxin-GA relationship across species and phylogenetic groups or across different tissue types and responses. Further, they do not touch on the ecological benefits of the auxin-GA interaction. These issues are discussed below as well as the need for the development of suitable experimental systems to allow this process to be examined.Key words: auxin, gibberellins, DELLA proteins, interactions, elongation     

Antagonistic roles of the N-terminal domain of prion protein to doppel

Prion protein (PrP)-like molecule, doppel (Dpl), is neurotoxic in mice, causing Purkinje cell degeneration. In contrast, PrP antagonizes Dpl in trans, rescuing mice from Purkinje cell death. We have previously shown that PrP with deletion of the N-terminal residues 23–88 failed to neutralize Dpl in mice, indicating that the N-terminal region, particularly that including residues 23–88, may have trans-protective activity against Dpl. Interestingly, PrP with deletion elongated to residues 121 or 134 in the N-terminal region was shown to be similarly neurotoxic to Dpl, indicating that the PrP C-terminal region may have toxicity which is normally prevented by the N-terminal domain in cis. We recently investigated further roles for the N-terminal region of PrP in antagonistic interactions with Dpl by producing three different types of transgenic mice. These mice expressed PrP with deletion of residues 25–50 or 51–90, or a fusion protein of the N-terminal region of PrP with Dpl. Here, we discuss a possible model for the antagonistic interaction between PrP and Dpl.Key words: prion protein, doppel, neurotoxic signal, neurodegeneration, neuroprotection, prion diseaseThe normal prion protein, termed PrP^C, is a membrane glycoprotein tethered to the outer cell surface via a glycosylphosphatidylinositol (GPI) anchor moiety.^1,2 It is ubiquitously expressed in neuronal and non-neuronal tissues, with highest expression in the central nervous system, particularly in neurons.³ The physiological function of PrP^C remains elusive. We and others have shown that PrP^C functionally antagonizes doppel (Dpl), a PrP-like GPI-anchored protein with ∼23% identity in amino acid composition to PrP, protecting Dpl-induced neurotoxicity in mice.^4–7 Dpl is encoded on Prnd located downstream of the PrP gene (Prnp) and expressed in the testis, heart, kidney and spleen of wild-type mice but not in the brain where PrP^C is actively expressed.^4,5,8 However, when ectopically expressed in brains, particularly in cerebellar Purkinje cells, Dpl exerts a neurotoxic activity, causing ataxia and Purkinje cell degeneration in Ngsk, Rcm0 and Zrch II lines of mice devoid of PrP^C (Prnp^0/0).^4,9,10 In these mice, Dpl was abnormally controlled by the upstream Prnp promoter.^4,5 This is due to targeted deletion of part of Prnp including a splicing acceptor of exon 3.¹¹ Pre-mRNA starting from the residual exon1/2 of Prnp was abnormally elongated until the end of Prnd and then intergenically spliced between the residual Prnp exons 1/2 and the Prnd coding exons.^4,5 As a result, Dpl was ectopically expressed under the control of the Prnp promoter in the brain, particularly in neurons including Purkinje cells.^4,5 In contrast, in other Prnp^0/0 lines, such as Zrch I and Npu, the splicing acceptor was intact, resulting in normal Purkinje cells without ectopic expression of Dpl in the brain.⁴The molecular mechanism of the antagonistic interaction between PrP^C and Dpl remains unknown. We recently showed that the N-terminal half of PrP^C includes elements that might mediate cis or trans protection against Dpl in mice, ameliorating Purkinje cell degeneration.¹² We also showed that the octapeptide repeat (OR) region in the N-terminal domain is dispensable for PrP^C to neutralize Dpl neurotoxicity in mice.¹² Here, possible molecular mechanisms for the antagonism between PrP^C and Dpl will be discussed.     

Hypoxia: A novel function for VIN3

VERNALIZATION INSENSITIVE 3 (VIN3) encodes a PHD domain chromatin remodelling protein that is induced in response to cold and is required for the establishment of the vernalization response in Arabidopsis thaliana.¹ Vernalization is the acquisition of the competence to flower after exposure to prolonged low temperatures, which in Arabidopsis is associated with the epigenetic repression of the floral repressor FLOWERING LOCUS C (FLC).^2,3 During vernalization VIN3 binds to the chromatin of the FLC locus,¹ and interacts with conserved components of Polycomb-group Repressive Complex 2 (PRC2).^4,5 This complex catalyses the tri-methylation of histone H3 lysine 27 (H3K27me3),^4,6,7 a repressive chromatin mark that increases at the FLC locus as a result of vernalization.^4,7–10 In our recent paper¹¹ we found that VIN3 is also induced by hypoxic conditions, and as is the case with low temperatures, induction occurs in a quantitative manner. Our experiments indicated that VIN3 is required for the survival of Arabidopsis seedlings exposed to low oxygen conditions. We suggested that the function of VIN3 during low oxygen conditions is likely to involve the mediation of chromatin modifications at certain loci that help the survival of Arabidopsis in response to prolonged hypoxia. Here we discuss the implications of our observations and hypotheses in terms of epigenetic mechanisms controlling gene regulation in response to hypoxia.Key words: arabidopsis, VIN3, FLC, hypoxia, vernalization, chromatin remodelling, survival